This invention relates to the manufacture of semiconductor electronic devices, and mote particularly, to a mold and a method for manufacturing a semiconductor device structure intended for surface mounting and being of a type which comprises a metal plate and a plastic (solidified resin) body joined to each other.
As is known, integrated circuits and other active electronic devices or components are built up on chips of a semiconductor material having a surface area on the order of a few square millimeters, and require for their connection to an external electric circuit special supporting, enclosing and electric interconnection structures. A typical structure suited for the purpose basically comprises a solid resin body enclosing a chip which is connected, by thin wires soldered to metallized regions specially provided on its surface, to corresponding electric conductors or terminal leads led out of the polymer body. With power integrated circuits, that is devices designed to operate on high currents, and therefore, liable to become heated to a significant extent, such structures also comprise a small metal plate through which a chip mounted thereon can transfer the heat generated during its operation to the environment.
In manufacturing the last-mentioned structures, the plate is blanked from sheet metal, e.g. copper; thereafter, a chip is secured on the metal plate either by soldering with a low-melt alloy such as a lead-tin alloy, or by cementing with a suitable adhesive such as an epoxy adhesive; a set of metal strips, intended to become the terminal leads for the device, are then blanked off thin sheet metal but still left joined to one another by interconnecting sections, and mounted to the plate in an electrically insulated manner therefrom; thin wires, usually of gold, are soldered, on the one side, to the metallized regions of the chip using a low-melt alloy, and on the other side, welded to the ends of the metal strips using a so-called "thermosonic" process wherein heat and ultrasound are applied simultaneously; thereafter, the assembly is placed into a specially provided mold, into which a material such as a thermosetting epoxy resin is then introduced in a liquefied state; on curing the resin, a structure is obtained which comprises a solidified resign body encapsulating the above-described elements excepting for one face of the metal plate and part of the metal strips, i.e. of the device terminal leads, and their interconnecting sections; the latter are then removed, along with any interconnecting sections present, as by blanking to yield the finished electronic product.
With devices intended for surface mounting, that is for attachment to a printed circuit board by welding the heat sink and the lead ends to specially arranged metal regions, all located on the same board surface, the leads are bent to have their free ends all lying in the plane which contains the exposed side of the heat sink. The soldering is performed by applying a low-melt solder layer onto the metal regions of the board and then arranging the circuit components on the board with their respective terminals, and in the instance of devices of the kind described above, with the exposed side of the heat sink, in contact with the metal regions, and by heating the board to melt the solder layers and thereby have the terminals and heat sinks soldered on cooling to their respective metal regions.
The soldered joint quality is then inspected for appearance and shape. With semiconductor devices which incorporate a heat sink, this inspection cannot be carried out (or can be carried out only imperfectly) because the border line between the heat sink and the solder are concealed from view. Further, checking that the heat sink is correctly aligned to the corresponding printed circuit area also becomes more difficult due to that border line being hidden from view.
To obviate such drawbacks, structures have been proposed wherein the metal plate of the heat sink has exposed edges on two opposed sides of the structure. To make such structures, however, a special, critical and troublesome, additional machining step is required to rid such opposed edges of the solidified resin that forms all around the plate during the molding process.
Another machining step which becomes usually necessary to prepare such structures for soldering is the removal of thin polymer layers which form on the outward side of the heat sink due to seepage during the molding process. To prevent such seepage, there are known molds having shaped interiors to present elements for pushing the metal plate against the bottom of the hollow wherein the solidified resin body of the structure is to be formed.
It is the object of this invention to provide a method for manufacturing packaged device structures without post-machining to remove solidified resin that has seeped onto the heat sink nor any other troublesome machining to remove solidified resin from the heat sink edges.
The present application provides a manufacturing method, for packaging integrated circuits, in which the back of the heat sink plate is left substantially free of solidified resin. Moreover, a clean metal surface is easily achieved at the edge of the heat sink plate. Note that this clean metal surface is adjacent to a relief in the solidified resin encapsulation body. Thus, the quality of the solder joint can easily be inspected at this location.
In the presently preferred embodiment, this is achieved using a mold which includes two plates which delimit at least one hollow adapted to receive the plate and to contain resin for forming the device body. Two elements of the mold push the down, at opposite ends of the top surface of the plate, to hold the plate against the bottom of the hollow. The hollow has two side extensions which are delimited by the side surfaces of the plate edges, thereby solidified resin projections are formed thereon which separate readily after the molding process. Thus, a structure is obtained wherein the plate has its bottom surface and two side edge portions fully exposed to allow optimum and controllable soldering to a printed circuit board.